During operation, a gas turbine engine compresses intake air, mixes the compressed air with fuel, and ignites the fuel-air mixture to produce combustive gasses, which are then expanded through a number of air turbines to drive rotation of the turbine rotors and produce power. Turbine nozzles are commonly positioned upstream of the turbine rotors to meter combustive gas flow, while also accelerating and turning the gas flow toward the rotor blades. A turbine nozzle typically assumes the form of a generally annular structure having a number of flow passages extending axially and tangentially therethrough. By common design, the turbine nozzle includes an inner endwall or shroud, which is generally annular in shape and which is circumscribed by an outer endwall or shroud. A series of circumferentially-spaced airfoils or vanes extends between the inner and outer endwalls. Each pair of adjacent turbine nozzle vanes cooperates with the inner and outer endwalls to define a different combustive gas flow path through the turbine nozzle. When assembled from multiple, separately-cast segments, which are mechanically joined together during engine installation, the turbine nozzle is commonly referred to as a “turbine nozzle ring assembly.”
The cross-sectional flow area across the turbine flow paths (referred to herein as the “turbine flow area”) has a direct effect on fuel efficiency and other measures of engine performance. Turbine flow area affects exit gas temperatures and metering rates through turbine nozzle, which impact the power conversion efficiency of the turbine rotor or rotors downstream of the nozzle. It is, however, difficult to manufacture a turbine nozzle having an ideal turbine flow area in an efficient, highly-controlled, and cost-effective manner. For example, in instances wherein a number of individual turbine nozzle segments are separately cast and assembled to produce a turbine nozzle ring assembly, it is often difficult to produce nozzle segments having tightly controlled inner dimensions due to uncertainties inherent in the casting process, such as dimensional changes resulting from metal shrinkage during cooling. While it is possible to fine tune part dimensions via the production of multiple molds in a trial-and-error process, such a practice is time consuming and may incur significant expense as each investment mold may cost several hundred thousand U.S. dollars to produce. It may be possible to adjust the turbine flow area, within certain limits, by cold working the vanes after casting to further open or close the flow path metering points. This solution is, however, less than ideal and may result in undesired distortion of the nozzle vanes, as well as obstruction of any cooling channels provided downstream of the metering points. Furthermore, even if a turbine nozzle is initially produced to have an ideal or near-ideal effective flow area, gradual material loss due to hot gas erosion and/or abrasion of the nozzle vanes and endwalls during operation can result in the undesired enlargement of the turbine flow area over time, which may ultimately necessitate replacement of the turbine nozzle.